The lifetime of carbon monoxide sensors is usually limited by the amount of deionized water in the canister. Typically, the electrochemical cell in the sensor is required to be hydrated in order to properly function; however, the deionized water within the sensor is typically lost to the environment. Since carbon monoxide sensors are typically located in conditioned spaces, and HVAC systems are generally designed to maintain a relative humidity of approximately 40% to 60% within the conditioned space, there is a significant driving force for the removal of the deionized water within the carbon monoxide sensor. As such, the size of the water reservoir within the carbon monoxide sensor should be large enough to support loss of water vapor to the environment over the life of the sensor; however, to accomplish this task, the reservoir dominates the size of the carbon monoxide sensor. There is therefore a need for an improved carbon monoxide sensor that contains a smaller reservoir, but is capable of maintaining the hydration of the electrochemical cell for the life of the sensor.